Method of controlling the density of sintered compacts



Patented Dec. 16, 1952 METHOD. OF CONTROLLING THE DENSITY F SINTERED COMPAGTS Raymond. S. Gurnick,'Lakewood, and Robert T. Joy, Willoughby; Ohio, assignors to Thompson Products, Inc., Cleveland, Ohio, a corporation of Ohio No Drawing. Application June 8,1950, Serial No. 166,970

'their vaporization temperatures to produce a mass that will observe the'laws of hydraulic flow, withthe metal'particles in suspension. This plastic mass is then formedinto a desired shape by means of processes such :as injection molding, extrusion, pressing and the like. The preformed articles thus produced are next placedv in a sintering'furnace and subjected to a relatively high temperature to drive off the binder by depolymerization. of" the organic polymer contained 'thereinp'withoutxleaving a carbonaceous residue.

The temperature of'the mass is progressively in.- creased until all of the binder is removedfby volatilization and'the metal particles are sintered together'into a coherent form.

It .has been observed that'it is difficult to control" the-amount of shrinkage which naturally occurs between the time that the binder-contain.- ing plastic mass is introduced into the sintering furnacev and the time of" removal of the finally sintered article 'fromthe furnace. This shrinkage is caused by the drawing together of the powdered metal particles. as the-plasticbinding material is being removed. The present invention now controls this shrinkage by regulation oi'rthe. rate of volatilization of the binder.

It is thereforean object of- .thepresent invention to provide a method of controlling the amount of shrinkage and hence the density of porous metal compacts.

Another object of the present'invention is :to provide a method for regulating, the rate of volatilization of the binder from the types of plastic masses previously described so that an article .ota predetermined density and predetermined physical dimensionscan be produced upon sintering.

Still another object of the present invention is to provide a method for producing sintered compacts in which the degree of shrinkage during volatilization of the binder and sintering of the metal powder can be controlled.

We have found the density and physical dimensions of a sintered metal compact may be controlled by controlling the rate of volatilization of the binder material as the plastic compact containing the metal particles suspended in a heat-fugitiveresin binder is heated through a temperature range where the binder is'volatilized, and up to the temperature used for sintering' the particles into a coherent form.

We havefound that the change in density upon heating such compacts is due to the shrinkage or drawing together of the powdered metal particles as the heat-fugitive binder is volatilized. If the binder is rapidly removed, the particles of powdered metal are left with relatively little surface contact and largevoid areas. If the binder is removed slowly, the particles are brought together leaving relatively few voids and a relatively large amount of surface contact. This phenomenon takes place through the solid, liquid and vapor phases of the heat-fugitive resin. The amount of shrinkage thus occurring is particularly'marked where relatively large amounts, on

the order of 35 to 50%, by volume of heatfugitive' binder material are used .in preparing: the plastic composition. Such relatively large amounts-of binder'material are used so-that the' plastic mass observes the laws of hydraul'icfiow, thus facilitating extrusion, and making possible a wide variation in the density and porosity of the ultimate article.

We have also found" that if the heat fugltive' binder is prevented from volatilizing, the binder exists as a liquid which'tends to coalesce; thereby bringing the metal particles closer together. As a result, when the article. is sintered, it .isvery dense,fhavin'g shrunk considerably from its molded dimension-sand is generally severely distorted from its previous form. On. the other hand, where"thevolatilization. of the bindermaterial is forced by rapidlyconductingaway the vapors as thesa m'e are volatilized. from the plastic composition, the metal particles are left at relatively great distances from each other. Consequently, during sintering, there is littletendencyfor them to bond together, and very'little shrinkageoccurs. However, the article is too porous for ordinary useand is structurally weak. I The control of' the rate of volatilization of the binder can be accomplished by adjusting one or more of three inter-related variables. The first of these is the control of the rate of heating of the preformed mass until the time that the sintering temperature is reached. The second is the rate of introduction of protective gas, such as hydrogen, cracked ammonia, or other inert gas, used to carry off the volatilized binder compounds.

The third is the control of the content of vaporized binder material in the atmosphere surround- 3 ing the mass being heated. In the case where a non-decomposing binder is used, this involves controlling the vapor pressure of the volatilized binder compound in the surrounding atmosphere.

A preferred binder is a thermally depolymerizable polyolefin, particularly polybutene, and other depolymerizable compounds such as polystyrene, polyacrylate, polymethacrylate, polyethylene, polypropylene, and polyvinylbutyl ether may be employed. These organic polymers decompose into lower molecular weight polymers or monomers during volatilization so that addition of such decomposition products to the atmosphere surrounding the article during volatilization of the binder effectively inhibits the volatilization of the binder at too rapid a rate.

Control of the rate of volatilization of the binder materials through one or more of the above described methods has been found to have a very substantial effect upon the density of the ultimate sintered article. For example, a plastic composition containing 100 parts by weight iron powder, 7 parts by weight polystyrene, and parts by weight microcrystalline wax, was shaped by means of injection molding and heated to a sintering temperature of 2100 F. in the presence of hydrogen. By varying the amount of polystyrene present in the furnace, sintered iron compacts having densities from 65% to over 90% of the theoretical density of iron were obtained. 1

The specific ranges of the variables to be used for any given material will, of course, vary with the density desired in the sintered article.

The volatilization of the binder and sintering of the article can be most conveniently accomplished by passing the plastic article after shaping through a furnace having areas of progressively increasing temperature. For example, the temperature at the inlet to the furnace may be slightly above the temperature of the mass after leaving the injection mold, normally on the order of 300 F., and the temperature at the outlet end of the furnace can be maintained constant at the sintering temperature of th metal or higher. In the case of iron, a temperature of 2100 F. is sufiicient for effective sintering. The rate of heating of the articles as they pass through the furnace can be conveniently controlled by varyin the velocity of the conveyor which carries the articles through the furnace. In th case of producing sintered iron compacts, We have found that a convenient heating rate for the compacts between the temperatures of 300 F. and 2100 F. is in the vicinity of F. per minute.

The control of the rate of the introduction of the sweeping gas, such as hydrogen, into the furnace may also be varied within wide limits depending on the density desired in the ultimate article. In producing iron compacts for most uses where substantial strength is desired, the gas velocity employed for the sweeping gas will be just sufficient to prevent diffusion of gaseous binder products into the sintering zone where they could decompose to form free carbon. On th other hand, where a substantially more porous article is to be produced, the protective gas can be introduced at velocities up to those which would create turbulence within the furnace. It will be understood that the furnace will normally be provided with means for con ducting away gaseous binder products picked up by the sweeping gas prior to reaching the higher temperature zones.

In the manufacture of sintered articles having a very high density, approaching the theoretical density of the metal itself, it is desirable to incorporate in the stream of protective gas passing through the furnace a supply of volatilized binder compound or products of its thermal decomposition. For example, in using a polystyrene binder, the control of the rate of volatilization of the binder can be accomplished by vaporizing a quantity of polystyrene and passing it into the furnace at a point upstream of the volatilization zone.

From the foregoing, it will be appreciated that we have herein provided a convenient method for accurately controlling the density of sintered porous compacts produced from a plastic preshaped mass having a large percentage of organic binder. The control of the mentioned variables can be accomplished to a very accurate degree to produce a sintered article having a density and physical dimensions within closely defined limits.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

We claim as our invention:

In the method of making porous sintered metal compacts of predetermined dimensions and density wherein a preformed plastic mass containing metal particles and a heat-depolymerizable polymeric binder is heated in a heating zone to a sintering temperature in the presence of a current I of protective gas, the step of adding to said current of gas a quantity of a depolymerization product of said binder, said depolymerization product comprising a material having a lower degree of polymerization than said polymeric binder in an amount effective to diminish the rate of volatilization of compounds resulting from the thermal depolymerization of said binder.

RAYMOND S. GURNICK. ROBERT T. JOY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,026,383 Coolidge May 14, 1912 1,636,763 Boving July 26, 1927 1,648,962 Rentschler et a1. Nov. 15, 1927 2,593,943 Wainer Apr. 22, 1952 FOREIGN PATENTS Number Country Date 616,839 Great Britain Jan. 27, 1949 

